Distillate hydrocarbon fuels are widely used for residential and industrial heating, diesel engines, jet engines, etc.. Generally such fuels are filtered just prior to use to remove any particulates which might clog or foul nozzles, strainers, injectors, etc..
Upon storage, distillate fuels are subject to contamination with water from multiple sources, such as condensation from the atmosphere or entrance of ground water into the storage vessel. The contamination frequently results in a discreet water phase. The aqueous contamination supports the existence and growth of bacteria and other microorganisms. These bacteria, etc., live in the aqueous phase present in the fuel and utilize the fuel as a source of carbon ( i.e., food), degrading the fuel in the process and producing dark-colored particulates of high molecular weight as a byproduct of this degradation. Some of the microorganisms responsible for this degradation process produce extra-cellular polymers or exopolysaccharides commonly known as "slime". The slime mediates the environment about the microorganism, contributing to the microorganism's ability to obtain food and protection necessary for survival and growth. This slime is quite sticky and tends to bind together the degraded fuel particles as well as other similar particulates formed in the bulk fuel by direct chemical oxidation during storage. The resulting dark, sticky mass is commonly referred to as "sludge".
When sludge is present in a steel tank, corrosion of the tank is also observed. Whether this is due to direct action of the microorganisms, physical/chemical conditions generated by the presence of the sludge itself (e.g., an oxygen differential situation), or by acids or other directly corrosive species generated by the overall process is not clear. What is known, however, is that accelerated corrosion of the steel occurs in the presence of biologically active sludge. The products of this corrosion(i.e., iron oxide or "rust") also accumulate in the sludge giving it additional bulk.
Occasionally, sludge mass is passed from the storage tank through the oil lines to the filter. This frequently occurs when fuel oil is added to the storage tank and the force of the incoming fuel oil disturbs the aqueous layer and the sludge is broken into smaller pieces that are carried out of the tank with the fuel. The sludge with its living organisms is captured by the filter, and the organisms proceed to function on the filter, eventually producing slime and sludge on/and throughout the filter media. The presence of this sludge greatly shortens the effective life of the filters used to remove particulate from the fuel. In addition to fouling the filter itself, slime has been shown to break off the downstream side of the filter and be carried by the fuel flow. This entrained sludge often causes fouling of strainers and nozzles.
In the past various biocides have been used in an attempt to prevent growth of bacteria and other microorganisms in distillate fuel being stored in tanks, and thus prevent formation of sludge. The use of such biocides has been found to be not as effective as desired in preventing fouling of filters, and, furthermore, requires replenishment of the biocide as the fuel is being replenished in the storage tank. More recently it has been discovered that the use of certain corrosion inhibitors such as imidazolines and diamines can inhibit the formation of sludge. The diamines are effective corrosion inhibitors and are known to leave solution and coat the metal walls of a container providing a protective shield against the liquid in the container. This type of surface active compound is referred to as a "filming" compound. The imidazoline, a corrosion inhibitor, is also a filming compound. It is not recognized in the trade that such filming compounds are also effective in preventing and dispersing sludge deposits.
Surprisingly, when tested, the imidazoline was found to be able to remove existing sludge and prevent sludge accumulation. It is our observation that the imidazoline is a biodispersant as it disperses existing sludge and prevents the formation of new sludge by dispersing slime as it is generated by the living organisms. However, when tested in combination with a film forming amine, it is found that this mixture is more effective than either component alone in removing and preventing sludge accumulation. Furthermore, additional tests have shown that the imidazolines of interest do not function as biocides to achieve elimination of sludge accumulation.